Akio Nishimoto

Remote refractive index difference meter for salinity sensor

A remote refractive index difference meter was designed and tested for use as a salinity sensor. the principle is to measure the difference between the real seawaters index and the standard seawaters. The meter is composed of a semiconductor laser, a partitioned water cell, and a solid position detector. The performance of the sensor is also described. >

Infrared absorption measurement using the photothermal deflection effect of thallium bromide iodide (KRS-5): Examination of basic characteristics

An infrared absorption measurement technique based on the photothermal deflection effect of thallium bromide iodide (KRS-5) is proposed. The optical system for infrared absorption measurement by this technique consists of a crystal block of KRS-5, an XY axis translation stage, an off-axis parabolic reflector, a He–Ne laser, and a position sensitive detector. To estimate the sensitivity of the system, its typical frequency characteristics and the thermal diffusion length of a block of KRS-5 were measured. The thermal diffusion length was measured over the chopping frequency range of 5–15 Hz. The accuracy of the measured thermal diffusion length was compared with that theoretically calculated using the physical constants of the thermal conductivity, density, and specific heat. The measured results agreed well with those calculated theoretically within ±4 μm. We also analyzed the frequency characteristics for a carbon black sample and estimated the detection limit of absorption that is characteristic for some irradiance levels at the sample surface. The results suggested that our proposed technique is useful for infrared absorption measurement.

In-situ spectral absorption profiler using optical fibers

Phytoplankton spectral absorption of light energy is one of the important factors to estimate primary production of the ocean. We have developed a system which measures in situ spectral absorption of suspended matter, mainly phytoplankton, by a modified opal glass technique. The system was designed to provide sub meter-scale profile of phytoplankton distribution without water sampling. The system consists of a halogen lamp as a white light source, a multi-channel spectrometer and submergible optical sensor unit. The collimating lens and diffuser constitute this unit. The optical fiber cables are employed to transmit the light between the sensor unit and the equipment on shipboard. The received light was measured by spectrometer which ranges from 400 nm to 800 nm with 1024-channel temperature stabilized linear photodiode array. Our preliminary experiment with the cultured phytoplankton (diatoms, flagellate and green algae) has shown that minimum detectable absorption coefficient is 0.01 m-1.

A Method of in situ Measurement for Counting and Sizing of Blue-Green Alga Particles by the Detection of Fluorescent Components at Two Wavelengths

To understand the ecosystem of lakes and the ocean, we have developed a new in situ measurement system for counting and sizing alga particles. The system is based on the fact that the laser induced fluorescence from the phytoplankton changes its spectrum according to the phycocyanin quantity which is abundant in blue-green algae. In the system, blue-green algae are discriminated by measuring the ratio of fluorescence components at the two wavelengths (655 nm and 685 nm) emitted from the phytoplankton irradiated with a 532 nm laser beam. The system gives particle size distribution for each alga in the mixed phytoplankton of blue-green and green algae.